any ir-reversable mod 99.9% of the time decreases the value of the guitar. Especially when your starting with a low end guitar anyway. Your talking about airbrushing which is going to DESTROY the resale value. I wouldnt expect half of what they usually go for unless its something amazing that someone just has to have when you sell.
how much? It totally depends on who you go to and what you want done. Paiting flames is going to be a whole lot less than a intricate mummy paintjob. You need to talk to a painter to find this out.